Hollow fiber membranes for use in treatment of aqueous fluids are widely used for industrial applications such as precision filtration, ultrafiltration, etc. and for medical applications such as dialysis, filtration, and diafiltration of blood. Recently, filtration by hollow fiber membranes has been widely employed in production process of beverages containing polyphenol such as wines and beer. As materials for these membranes, there are used cellulose, cellulose acetate, polyethylene, polysulfone, poly(vinylidene fluoride), polycarbonate, polyacrylonitrile, etc.
For example, the following points are given as essential properties required for such hollow fiber membranes:
(1) a high removing ability to a substance to be removed,
(2) a high permeability to a substance to be permeated (i.e., fractionation performance as integral properties (1) and (2)),
(3) a high permeability to a fluid to be treated (permeability) (i.e., membrane performance as integral properties (1), (2) and (3)),
(4) a sufficiently high strength to hardly cause rupture or leakage (strength),
(5) no aging degradation in fractionation performance (fractionation sustainability), and
(6) no aging degradation in permeability to a fluid to be treated (permeability sustainability) (i.e., membrane performance sustainability as integral properties (5) and (6)).
Additionally, the following properties are required for hollow fiber membranes for use in filtration of beverages, which are required to be used over a long period of time:
(7) excellent recovery of fractionation performance by washing (i.e., fractionation recoverability), and
(8) excellent recovery of permeability by washing (i.e., permeability recoverability) (membrane performance recoverability as integral properties (7) and (8)).
Conventionally, most of hollow fiber membranes have been developed from the viewpoint of improvement of filtration performance as the property (3), and other properties sometimes have been sacrificed. General measure to improve the permeability of membranes is to increase the pore diameters of the membranes, which simultaneously induces degradation in the fractionation performance and strengths of the membranes.
Hollow fiber membranes are roughly classified by their membrane structures, to symmetrical membranes in which the pore diameters are substantially not changed in the thickness directions, and asymmetrical membranes in which the pore diameters are continuously or discontinuously changed and thus differ at the inner surfaces, inner portions and outer surfaces of the membranes. The symmetrical membranes show high resistance to the flow of fluids at their entire thicker portions thereof when used for filtration, and therefore, it is difficult to achieve a large flow amount, and the membranes are liable to clog due to solutes (i.e., substances to be removed).
Both the surface layer effect due to the pore diameter of the membrane surface and the depth effect due to the wall portion of the membrane contribute to removal of a subject substance through filtration of a fluid. Separation depending on mainly the depth effect is expected to achieve sharp fractionation performance, but is difficult to achieve a larger flow amount, since this separation requires for a membrane to have a certain thickness. Consequently, a membrane of this type has a defect in that the flow amount tends to decrease with time because of the clogging of the membrane due to a substance to be removed. The contribution of the depth effect is relatively large in the above-described symmetrical membrane, and thus, this defect is considered to be easily revealed in the symmetrical membrane.
Under such circumstances, there have been studied asymmetrical membranes having thereon thin skin layers which mainly determine the fractionation performance and permeability of the membranes. Patent Publication 1 discloses an aromatic polysulfone hollow fiber membrane which has, in its inner portion, pores in the smooth shapes of ellipse or circle, having a maximum longer axis of at least 0.1 μm, and which has a skin layer on its outer surface and has no macro void in its section. According to this technique, the pores in the shape of ellipse or circle make it possible to achieve sharp fractionation performance to thereby decrease a local force applied to a blood cell component during filtration of blood, so that a problem such as hemolysis or the like can be solved. While this effect surely can be expected by controlling the shapes of the pores, the study on the structure of the sectional portion of the membrane is insufficient, and especially, careful and keen attentions are not paid to the sustainability and recoverability of the membrane performance.
Patent Publication 2 discloses a hollow fiber type precision filtration membrane which comprises aromatic polysulfone and polyvinyl pyrrolidone and which is specified in polyvinyl pyrrolidone content, membrane structure and strength at break. In this membrane, the pore diameter of the inner surface is controlled in order to improve the permeability of the membrane. Specifically, the pore diameter of the inner surface should be smaller than the diameter of a substance which is inhibited from passing by filtration: that is, the pore diameter is from 0.01 to 1 μm, preferably from 0.05 to 0.5 μm. However, an error in measurement of the pore diameter tends to be larger, depending on the shape or size of the pore. Therefore, it is essential that the diameter of a substance to be inhibited from passing in filtration should be from 0.015 to 1 μm. It is also described in this publication that the strength at break of this membrane is at least 50 kgf/cm2, since a rupture strength of lower than 50 kgf/cm2 will frequently cause leakage. Thus, this membrane is not suitable for practical use. It is also described that, in case of blood as a fluid to be filtered, the concentration of hydrophilic polyvinyl pyrrolidone in the inner surface of the membrane is from 20 to 45% by weight in order to inhibit the adsorption of blood plasma protein. According to this technique, this membrane is developed in consideration of high strength, high water-permeating performance (high permeability) and prevention of clogging (fractionation performance sustainability), and in fact, these subject matters are considered to be solved to a certain degree. However, there is no description about membrane performance sustainability and membrane performance recoverability by washing which are required when this membrane is used as a membrane for obtaining tap water or a membrane for treating beverages over a long period of time. Thus, this membrane is insufficient in study of these problems.
Patent Publication 3 discloses a semipermeable membrane with a multi-layer structure, which comprises a ε-caprolactam soluble polymer, and comprises a separation layer A having a separation limit of from 500 to 5,000,000 Dalton, a support layer B having a hydrodynamic resistance negligibly small as compared with those of the layers A and C, and the layer C having pores with diameters larger than those of the pores of the separation layer A and smaller than those of the pores of the support layer B. Described as the subject matter to be achieved by this technique is to provide a membrane which has a designated separation limit and can have a low, medium or high permeability as required, by accurately controlling the separation limit and hydrodynamic permeability, while accurately controlling the hydrodynamic permeability independently of the separation limit. However, the strength, membrane performance sustainability and membrane performance recoverability of this membrane are taken out of consideration.
Patent Publication 4 discloses a membrane having a structure similar to the above-described membrane. This membrane has pores with a pore diameter of 500 nm or less at a layer in the proximity of the membrane inner wall surface and has a pore distribution at its section in the membrane thickness direction, which distribution has at least one pore with a maximum diameter of a specified value. This technique substantially relates to a medical membrane excellent in biocompatibility, which is intended to inhibit a high molecular weight protein from infiltrating the membrane and to decrease a contact area of the membrane to the high molecular weight protein to thereby improve the biocompatibility, by forming a minute inner surface which is to contact blood. A minute structure in the proximity of the outer surface of the membrane, following the maximum pore portion of the section of the membrane, is formed to inhibit infiltration of an endotoxin fragment from the outer surface of the membrane. That is, the minute-coarse-minute structure of the membrane is necessary as a blood-treating membrane for its substance-removing ability, biocompatibility and inhibition of infiltration of endotoxin. However, any relationship with other properties such as membrane performance sustainability and membrane performance recoverability is not described.
Patent Publication 5 discloses a method for removing polyphenol from a liquid containing polyphenol by using, in this liquid, polyvinyl polypyrrolidone (hereinafter referred to as PVPP) and a substance having a laccase enzymatic activity in combination, from the viewpoint of treatment of polyphenol. This technique is characterized by the steps of polymerizing polyphenol so as to have a size liable to be adsorbed onto PVPP, using the substance having the laccase enzymatic activity; adsorbing the polyphenol onto PVPP; and removing the adsorbed polyphenol. In this technique, a step of removing the PVPP used for treatment is indispensable, and therefore, the steps of this method is complicated.
Patent Publication 6 discloses a technique for efficiently clarifying beer by a single step of treatment using a composition containing silica xerogel and crosslinked polyvinyl pyrrolidone. However, this technique also requires a step of removing the composition after the clarifying treatment.
Patent Publication 7 discloses a method for stabilizing a beverage containing a hazing material, the method comprising the steps of bringing the beverage into contact with a water-insoluble porous hydrophilic matrix, and recovering the beverage from the matrix. In this publication, a membrane is given as an example of the forms of the matrix. However, there is no description about the structure and features of the membrane in case where the membrane is selected as the matrix. This publication substantially relates to a treating method using a column charged with beads type matrix. In this technique, the consideration of separation of a permeated substance and a substance to be removed, by way of membrane filtration, is insufficient.
As has been described above, there is not found in the known techniques, any technical idea to obtain a membrane particularly suitable for use in treatment of a beverage containing polyphenol, by taking into consideration the interaction of the membrane with polyphenol, the structure of the membrane and the content of a hydrophilic polymer in combination.
Patent Publication 1: JP-B-07-022690/1995
Patent Publication 2: Patent Registration No. 3594946
Patent Publication 3: JP-A-11-506387/1999
Patent Publication 4: JP-A-09-047645/1997
Patent Publication 5: JP-A-2004-267177/2004
Patent Publication 6: JP-A-2002-515236/2002
Patent Publication 7: JP-A-10-042852/1998